1. Field
The field relates to a pixel and an organic light emitting display device using the same and, more particularly, to a pixel and an organic light emitting display device that can display an image having uniform luminance.
2. Description of the Related Technology
Various flat panel display devices having reduced weight and volume when compared with a cathode ray tube have been developed. Examples of the flat panel display devices include a liquid crystal display device, a field emission display device, a plasma display panel, an organic light emitting display device, etc.
The organic light emitting device displays an image by using an organic light emitting diode that emits light by recombining holes with electrons. Such the organic light emitting display device has an advantage of being driven at low power consumption while having rapid response speed.
FIG. 1 is a circuit diagram showing a pixel of an organic light emitting display device.
Referring to FIG. 1, the pixel 4 of the organic light emitting display device includes an organic light emitting diode OLED and a pixel circuit 2 for controlling the organic light emitting diode OLED by being connected to a data line Dm and a scan line Sn.
An anode electrode of the organic light emitting diode OLED is connected to the pixel circuit 2 and a cathode electrode of the organic light emitting diode OLED is connected to second power ELVSS. The organic light emitting diode OLED generates light having luminance according to the amount of current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED according to a data signal supplied from the data line Dm when a scan signal is supplied to the scan line Sn. For this, the pixel circuit 2 includes a second transistor M2 connected between first power ELVDD and the organic light emitting diode OLED, a first transistor M1 connected between the second transistor M2, the data line Dm, and the scan line Sn, and a storage capacitor Cst connected between a gate electrode and a first electrode of the second transistor M2.
A gate electrode the first transistor M1 is connected to the scan line Sn and the first electrode of the first transistor M1 is connected to the data line Dm. In addition, a second electrode of the first transistor M1 is connected to one terminal of the storage capacitor Cst. Herein, the first electrode is either of a source electrode and a drain electrode and the second electrode is an electrode other than the first electrode. For example, when the first electrode is the source electrode, the second electrode is a drain electrode. The first transistor M1 connected to the scan line Sn and the data line Dm is turned on when the scan signal is supplied from the scan line Sn, such that the data signal supplied from the data line Dm is supplied to the storage capacitor Cst. At this time, the storage capacitor Cst is charged with voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is connected to one terminal of the storage capacitor Cst and the first electrode of the second transistor M2 is connected to the other terminal of the storage capacitor Cst and to the first power supply ELVDD. In addition, a second electrode of the second transistor M2 is connected to the anode electrode of the organic light emitting diode OLED. The second transistor M2 controls the amount of current that flows to the second power ELVSS via the organic light emitting diode OLED from the first power ELVDD according to a voltage stored in the storage capacitor Cst. The organic light emitting diode OLED generates light corresponding to the amount of current supplied from the second transistor M2.
However, the pixel 4 of the organic light emitting display device of FIG. 1 cannot display an image having uniform luminance across many pixels. More specifically, threshold voltage of the second transistor M2 (driving transistor) included in each of the pixels 4 varies somewhat for each pixel 4 because of process deviation, and other effects. When the threshold voltage of the driving transistor varies among the pixels, even though a data signal corresponding to the same gray scale is supplied to the pixels, light having different luminances is generated by the pixels.
In order to solve the problem, there is proposed a structure in which transistors are additionally formed in each of the pixels 4 in order to compensate for the threshold voltage variation of the driving transistor. There are known pixels which use six transistors and one capacitor in each of the pixels 4 to compensate for threshold voltage variation. However, when six transistors are included in each of the pixels 4, the pixel 4 is complicated. In particular, malfunction probability is increased by the large number of transistors included in the pixels 4, such that a yield is deteriorated.